As I rewrite Prime Base I'm considering using a book code to provide clues to the players. To be effective the code needs to reference a text that the characters will be able to access but that it is far less likely that the bad guys will have handy. I am considering the semi official goals of the Project.

Help the People
Contact Other Morrow Teams
Try and stay alive

This has acehilmnoprstvw, so doesn't have every letter. It might be rejected out of hand by none project cryptographers for that reason alone. Plus although it is a long running in character in game set of statements I'm not sure it would be written down that much in Project documentation.

We play entirely different types of games with entirely different types of players

Undoubtedly. I was under the impression you were making a re-write of PF08 for sale by Timeline?

I say this as I am paraphrasing (earlier) the "Play of the game notes" found at the end of the module. There is distinctly mentions that not one of the playtest groups discovered the real Prime Base.

I try to write for the teens and the 20s. That player base wants action. PF01, for example, is popular and rightly so. Not for us sedentary 40-60 year olds with selective recollection of the cold War. Such as mine.

"PACK MY BOX WITH FIVE DOZEN LIQUOR JUGS" was the song everybody learned in the Morrow Project boot training camp. Why? Because it was the key to decoding messages. All letters of the English alphabet used at least once in a small sentence of 32 letters.

I don't think the Project would have such a code established already simply because it would be so vulnerable to either a traitorous Project member (and there will be some) or extraction through torture or other means, and it is unlikely that a suitable code phrase could wind up in Morrow training materials by accident. But it could be possible that someone involved with training could have had an interest in cryptography and slipped such a code in without ever really expecting to use it. In the desperation surrounding the fall of Prime Base, it is not inconceivable that someone could have used the code not having any better alternative. If that is the case the code should be something non-descript, so perhaps the code is already a mnemonic for something else or perhaps part of the motto.

An alternative would be to do it like a Playfair cipher using keywords like "Morrow" or actually hide the code words somewhere in the base. Or use an Ottendorf cipher that uses the Morrow Field Manual to decode. If Teams were exposed to such tools during their survival training they could readily use them here.

In one game I was part of, as Prime was dying, one of the programmers who were developing the "last app" had a bright idea.

Prime location and back story (at least as they understood what was killing them) were transmitted to all regional supply bases. IF a level 4 or above Morrow person put in his/her card, then the story would unfold. The location was NOT latitude x longitude. Some physical clues were put in. it was up to Morrow people to get to location and deduce the exact spot.

Something like:

In NW Nevada, look for the windmill where it should not be.

Only problem, per Prime Base module, the windmill on top of the mesa had been blown down by the nuke in the valley. It did leave a baked MP in the mud.

It took our team FOREVER to even get close. NW Nevada is HUGE!!! We got lucky by running into a small heard of buffs. Buffalo meat is very lean and has an interesting flavor. We rigged up a distillery using heat from the engine to purify water. We kept our canteens and water storage units as full as possible.

What finally got our attention was the slightly radioactive lake behind the
landslide wall. The auto nav had no clue what this lake was doing HERE ??!!??!!

We EVENTUALLY found our way into Prime. We had to pull out after a week of wandering around and NOT finding how to restart Prime.

As the game wound down, we had setup a small fire base NEAR PRIME. We were looking for other Morrow people to help us.

The Teams don't need a code, they need a Shibboleth. The Teams should know 2 things that positively identify Prime Base, then let the Grand Deception reveal one and hide the other. That way, a Morrow Team finding the site would see the first thing, but not finding the second should realize that some part of the facility has not yet been discovered.

For example, perhaps Teams (or at least Team Leaders) are told that PB is the only facility active during the war and is also a repository of art and artifacts. In the base they find plenty of evidence that it is a large Morrow facility and was active during the war, but no Monets. Where are they??

The Teams don't need a code, they need a Shibboleth. The Teams should know 2 things that positively identify Prime Base, then let the Grand Deception reveal one and hide the other. That way, a Morrow Team finding the site would see the first thing, but not finding the second should realize that some part of the facility has not yet been discovered.

For example, perhaps Teams (or at least Team Leaders) are told that PB is the only facility active during the war and is also a repository of art and artifacts. In the base they find plenty of evidence that it is a large Morrow facility and was active during the war, but no Monets. Where are they??

Excellent idea. This should be obvious even without doing it on purpose though. There are a lot of things missing in the rump base above the deception level.